Field of the Invention
The invention relates to a hydraulic control rod drive for a nuclear reactor, in particular for a heating reactor, with a piston/cylinder drive for a respective control rod and a reactor plenum enclosing the drive.
Heating reactors are designed for a relatively low primary pressure of, for instance, 15 bar; the primary medium or working fluid circulates in the reactor pressure vessel by the natural circulation principle. The fuel element bundle of such a heating reactor and the flow box are preferably embodied mechanically separate from one another, whereby the flow box is as long as dictated by the stack height necessary for natural circulation. Four fuel element bundles respectively are combined within such a box, the core box, including the associated control rod with drive. The control rods are moved within cross-like guides, which are disposed in the box of the reactor core. The integration of the control rod drives in the primary system necessitates a drive system which is compatible with the primary coolant, i.e. with water. The low nuclear power density due to the natural circulation principle and the corresponding control requirements make it possible to use a hydraulic drive which is operated with primary water as the working fluid. The fuel element bundle and the actual control rod are thereby preferably embodied as they are known from boiling water reactors. In general, a heating reactor has characteristics of a boiling water reactor as well as of a pressurized water reactor.
A hydraulic control rod drive for a nuclear reactor is known from German Published, Non-Prosecuted application 34 30 929, entitled "Boiling Water Reactor Control Rod Drive Using Coolant as Hydraulic Fluid With Ultrasonic Locating Probe for Rod Position". In that device, the position of the control rods is determined by way of an ultrasonic measurement device with a supersonic transducer, which serves as an ultrasonic sender and receiver. The transducer is disposed in a linear channel between the control rod drive and the upper fill level of the reactor coolant.
The invention starts out with the appreciation that gas bubbles in the drive system can under certain circumstances lead to oscillations of the control element and thus to undesireable reactivity fluctuations. The above-mentioned prior art teaching does not provide for venting any gas bubbles from the drive system.
Regarding a venting opening provided in the context of this invention, working fluid leaks through the opening which corresponds approximately to the core inlet temperature. Due to heat conduction and heating by gamma radiation absorption, the working medium temperature in the guide shaft of the associated fuel element may be up to about 30.degree. C. above the temperature of the partial fluid currents through the venting opening. Accordingly, eddies with varying water density can form in the opening region of the venting openings, which eddies impair the ultrasonic distance measurement.
It is accordingly an object of the invention to provide a hydraulic control rod drive for a nuclear reactor, and particularly for a heating reactor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and taking the afore-mentioned dangers into account. The invention is based on the object to improve the control rod drive of the above-mentioned kind in its operational functionality in a sense that, on the one hand, the piston-cylinder drive is effectively vented of gas bubbles and that, on the other hand, the partial fluid flows exiting from the venting opening cannot impair the ultrasonic distance measurement.
With the foregoing and other objects in view there is provided, in accordance with the invention, in a nulcear reactor having a fuel assembly with a control rod and a reactor plenum enclosing the fuel assembly, a hydraulic control rod drive, comprising a movable cylinder having control rod elements disposed thereon; a stationary piston rigidly mounted in the fuel assembly, the piston having a bottom, an open top, and an axial opening formed therein for allowing communication between the bottom and the open top; the cylinder being disposed coaxially around the piston and defining a gap therebetween so as to allow axial movement of the cylinder; means for supplying a working fluid to the piston for lifting, lowering or suspending the cylinder; means for measuring a vertical displacement of the cylinder; and means for venting the gap between the piston and the cylinder.
In accordance with another feature of the invention, the control rod drive measuring means include an ultrasonic reflector disposed on the cylinder, and an ultrasonic transducer rigidly mounted above the cylinder in alignment with a longitudinal axis of the cylinder.
In accordance with again another feature of the invention, the venting means are in the form of a venting channel formed in an upper end of the cylinder for allowing working fluid with gas bubbles to escape from the cylinder.
The invention is embodied in a hydraulic control rod drive for a nuclear reactor, in particular a heating reactor, of the above-mentioned structure, which provides the following further features, in accordance with the invention, for solving the object as presented:
a) besides a cylinder, a piston is also formed as a hollow body,
b) a first one of the two hollow bodies is stationary and serves to supply the working fluid,
c) the second one of the two hollow bodies is coaxially mounted around the first hollow body with an annular gap in between and is movable upward and downward and forms a carrier body for control elements of the control rod,
d) the working fluid can be supplied into the two hollow bodies via a supply channel in the lower region of the first hollow body for mass flow-dependent lifting, lowering or suspending the second hollow body and can be removed from the inner space of the two hollow bodies via a throttle passage, preferably an annular gap,
e) an ultrasound reflector is attached at the upper, head end of the second hollow body, which reflector forms a positional measurement system with an ultrasonic measurement path together with an ultrasonic transducer rigidly mounted above and remote from the second hollow body,
f) the second hollow body has a venting channel configuration in the region of its upper end for venting the second hollow body, which configuration opens into the reactor plenum at a security distance a1, a2 from the ultrasonic measurement path which is great enough such that the ultrasonic measurement remains virtually unaffected by density fluctuations in the working fluid.
The advantages attained with the invention are mainly seen in the fact that, on the one hand, an effective venting of the inner space of the control element, or hollow cylinder/hollow piston is provided and, on the other hand, the partial venting currents associated with this venting can have no or virtually no disadvantageous effect on the ultrasonic distance measurement of the control element.
In a preferred embodiment of the invention the first of the two hollow bodies is preferably a hollow piston open on the top and, correspondingly, the second hollow body is a hollow cylinder, which covers the hollow piston coaxially and with an annular gap, whereby the axial position of the hollow cylinder relative to the hollow piston can be determined by the amount of working fluid supplied through the hollow piston into the interior of the hollow cylinder.
According to an advantageous further development of the invention and in the case when the control elements are embodied as cross-shaped absorber plates, a cross-shaped attachment part for forming the venting channel configuration is attached at the head end of the hollow cylinder. Radial channels of the venting configuration originate from a central channel part communicating with the hollow cylinder interior, extend through the cross legs of the attachment part, and open into the reactor plenum approximately in the region of the absorber plate tips.
According to another advantageous embodiment of the hydraulic drive, whereby cross-shaped absorber plates are likewise assumed, there is provided for the venting channel configuration to be formed by a head plate whose diameter corresponds approximately to that of the hollow cylinder and which has a venting channel configuration disposed therein which is approximately T-shaped in axial section. The outlet pipes with outlet openings are inserted into the channels and the outlet openings are disposed at a security distance from the ultrasonic measurement path.
When the pressure of the hydraulic working fluid within the piston/cylinder drive relative to the reactor plenum is not more than about 1.5 bar, it is suggested that the channels of the venting channel configuration opening into the reactor plenum extend at a slight rise of, for instance, 5.degree. to 10.degree. with the horizontal.
In accordance with a concomitant feature of the invention, the control rod drive is embodied such that the first hollow body on its outer circumference and the second hollow body on its inner periphery are provided with first annular protrusions and recesses and with cooperating second annular recesses and protrusions, wherein an annular gap is defined between the first protrusions and recesses and the second recesses and protrusions. The annular gap forms a throttle passage for the working fluid from the inner space of the two hollow bodies into the reactor plenum.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a hydraulic control rod drive for a nuclear reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of the specific embodiment when read in connection with the accompanying drawings.